1. Field of the Invention
This invention relates to a wheel crane such as a rough terrain crane.
2. Description of the Prior Art
An exemplary one of conventional cranes is shown in FIG. 3.
Referring to FIG. 3, the crane shown is a rough terrain crane and includes a lower running body 1, an upper pivotal body 2 carried on the lower running body 1, and a telescopic boom 3 mounted for pivotal up and down movement around a boom foot pin 3a on the upper pivotal body 2.
The upper pivotal body 2 includes a main frame 2a and a winch frame 2b, and a base end portion of the telescopic boom 3 (boom foot) is mounted for pivotal motion at a rear end portion of the main frame 2a by means of the boom foot pin 3a. Meanwhile, a pair of winch drums 4 and 5 are disposed in an offset condition in the forward and backward directions in the inside of the winch frame 2b, and a pair of guide sheaves 6 and 7 for guiding ropes R.sub.1 and R.sub.2 drawn out from the drums 4 and 5 to the rear face side of the boom 3 by way of the base end side of the boom 3 are disposed in a corresponding relationship above the winch drums 4 and 5, respectively. A counterweight 8 is mounted on a rear end face of the winch frame 2b.
With such a wheel crane as described above, it is desired to employ an elongated boom in order to increase the operation capacity, but since the extent of a forward extension of the boom 3 when the wheel crane runs on a public road is restricted by regulation, the length of the boom cannot be made long without restriction. Thus, it may be advisable to position, as means for making the length of the boom 3 as long as possible under the restriction of the law, a boom foot mounting point on the upper pivotal body 3, that is, the boom foot pin 3a, at a position of the upper pivotal member which is as rearwards as possible. In this instance, however, there are the following problems.
In particular, if the boom foot mounting point is displaced rearwardly, then the guide sheaves 6 and 7 are also displaced rearwardly, and consequently, the winch drums 4 and 5 must also be displaced rearwardly. Accordingly, a rear end of the upper pivotal body 2, that is, a rear end of the counterweight 8, is displaced rearwardly by a distance equal to the distance over which the boom foot mounting point is displaced rearwardly. However, with the conventional wheel crane, the distance l.sub.1 from the base end of the boom 3 to the rear end of the pivotal body 2 is great due to the fact that,
(a) since the guide sheaves 6 and 7 and the winch drums 4 and 5 are both disposed in a juxtaposed relationship in the forward and backward directions, the winch frame 2b for accommodating the drums 4 and 5 has a great length in the rearward direction; and
(b) since the boom foot mounting point (boom foot pin 3a) on the pivotal body 2 is located intermediately in the vertical direction when the boom 3 is in a horizontal position at the base end portion thereof, a great distance must be provided between the base end of the boom 3 and the front guide sheave 6 in order to avoid possible interference between them when the boom 3 is pivoted uprightly.
Therefore, if the boom foot mounting point is displaced rearwardly further, then the distance l.sub.2 from the center 0 of pivotal motion of the boom 3 to the rear end of the pivotal body 2, that is, the maximum diameter of pivotal motion of the upper pivotal body 2, becomes excessively great, and consequently, the operability in a narrow location is deteriorated. This is a very serious drawback with a rough terrain crane with which great importance is attached to the operability at a narrow location.
By the reason described just above, it is considered impossible with a conventional wheel crane to displace the boom mounting point rearwardly from a current position.
Further, as a result, the boom foot mounting point is currently positioned forwardly of the center of a rear axle 1a of the lower running body 1 as shown in FIG. 3. Since, according to the layout, there are several problems in that the safety in running is deteriorated by imbalance between loads to the front and rear axles upon running and so forth, a conventional crane employs, as a countermeasure, means for increasing the thickness of the counterweight 8 to increase the weight of the counterweight 8. However, since this increases the maximum radius of pivotal motion, it is necessary to displace the winch box 2b forwardly by a distance corresponding to the amount of increase in thickness of the counterweight 8. This is also a factor which obstructs displacement of the boom foot mounting point.
From such circumstances as described above, it is considered actually impossible with a conventional wheel crane to increase the length of a boom further than the current length of the boom.